SHODH SANGAM -- A RKDF University Journal of Science and Engineering

ISSN No. 2581-5806 http://www.shodhsangam.rkdf.ac.in Vol.-03, No.-04, July-2020, Page 25

Voltage Stability Analysis of Grid Connected

Photovoltaic Power System Mr. Shahid Akhtar Iqbal Ahmed, Prof. Varsha Mehar

Research Scholar, Bhabha College of Engineering,

RKDF University, Bhopal, Madhya Pradesh, India

shawnak07@gmail.com

varshamehar86@gmail.com

Abstract — Performance of the power system can be

improved by integrating renewable energy sources into

the grid. In this paper various impacts of PV

penetration on the performance of an IEEE 14 bus

system is studied. Continuation power flow (CPFLOW)

is performed on the system without PV system and with

gradually increasing the solar power penetration. The

solar plant is installed at bus 9 and 14 as these are the

weakest buses. The simulation analysis was performed

by PSSE (Power System Simulator for Engineers).

Results of this research show the benefits of

introducing solar power to existing power grid. It

increases the power handling capacity of the system

and voltage instability was occurring at higher loading

factor. Finally, the steady state bus voltages became

higher than without PV penetration and at a certain

point it exceeds the permissible upper voltage limit,

and that’s the maximum point up to which, the

penetration can be done.

Keywords: Voltage stability, photovoltaic system,

PSSE, continuation power flow.

I. INTRODUCTION

Increased production of goods per head, increased

prosperity and urbanization, rise in per head

consumption, and easiness in energy access are the

factors that are responsible for the increase in the total

demand of electricity by a significant extent. Having a look at the difference of electricity demand and supply,

huge quantities of coal and furnace oil are being used.

These usages need to be reduced, as these are leading to

tremendous costs in the form of subsidies and increment

in the country’s dependency on imports. Renewable

energy sources have the ability to make a noteworthy

contribution in these areas. Due to all of these,

renewable energy needs to be studied and utilised to a

great extent [1]. Therefore, commissioning of solar

power units in the existing grid give rise to problems

like, violation of bus voltages beyond the stipulated grid

limits, power congestion, abnormal system losses and voltage instability. Solar power has an exceptionally

good potential for providing electrical energy that is free

& non-polluting. Its effectiveness as an electricity

supply source has encouraged ambitious targets for solar

PV system in many countries around the world.

II. SOLAR PHOTOVOLTAIC SYSTEM

The most important component of the photovoltaic

(PV) system is the solar panels that generate electric

power by the direct conversion of the sun’s energy into

electricity. The solar panels are mostly made with semiconductor material, with Silicon (Si) being widely

used. Materials like Gallium (Ga) and Aluminium (Al)

have better conversion properties and recently they are

increasingly finding their application. The components

of the PV system include the electronic devices to

interface the PV output and the AC or DC loads.

A major challenge in maximum utilization of solar

cells for power generation is improving cell efficiency

and optimizing energy extraction. The solar cell can

generate maximum power at a specific operating point,

but that operating point varies depending on the

atmospheric conditions. This varying output limits the ability of utilities to predict output power at a given time

for that location and thus creating problem in scheduling

their generation. The optimum operating point for the

cell to generate the maximum power can be determined

from the I-V (current-to-voltage) characteristic.

The voltage-current characteristic of a solar cell has

two different regions:

The current source region

The voltage source region

Fig. 1 Typical I-V characteristic of a solar cell

In the first region of the I-V characteristics, the solar

cell has high internal impedance and the output current

flows with a constant value while voltage keep on

increasing, on the other hand the terminal voltage

remains constant over a wide range of output current

and internal impedance is low in the later region.

SHODH SANGAM -- A RKDF University Journal of Science and Engineering

ISSN No. 2581-5806 http://www.shodhsangam.rkdf.ac.in Vol.-03, No.-04, July-2020, Page 26

Fig. 2 Solar PV System Connected to Grid

Theory of maximum power transfer states that,

“maximum power is delivered to the load when the

source internal impedance and the load impedance

become exactly same” [11]. Therefore, the impedance of

the solar cell at the output side is matched impedance of

the load. This will ensure operation of the solar cell at

the optimum level. Thus the maximum power operating

point can be maintained by controlling either the voltage

or output current or both. Since environmental

conditions like temperature and irradiance vary the

maximum operating point, maintaining the operating point at the optimum point (MPP) becomes

unpredictable, resulting in variation in the output power.

An MPPT is thus employed to accomplish the task.

Most MPPT controllers are based on the buck converter

(step-down), boost converter (step-up) or buck-boost

converter setup.

III. CONTINUATION POWER FLOW

Singularity of the Jacobian matrix of power flow

equation occurs at voltage stability limit. Continuation

power flow takes control of this problem. CPFLOW

executes successful load flow solutions in accordance to a load scenario.

It comprises of prediction and correction steps. From

a known base solution, a tangent (known as predictor) is

employed so as to estimate next solution for an outlined

pattern of load increase. The corrector step then

determines the precise solution using Newton- Raphson

technique employed by a traditional power flow.

afterward a brand new prediction is formed for an

outlined increase in load based upon the new predictor.

Then corrector step is applied. This process goes until

sensitivity is reached. The sensitive point is that the

point where the tangent vector is zero. The flow chart of predictor-corrector scheme is illustrated in Figure 4.

Fig. 3 Illustration of prediction-correction steps

Fig. 4 Flow chart of CPFLOW

IV. MODELLING OF THE COMPONENTS IN PSS/E

PSS/E is capable of performing both steady state analysis and transient analysis. The dynamic simulation

feature is to be used here because of its capability to

simulate the transient behavior of each and every

component used in the system, during a fault and post

fault conditions. PSS/E consist of large no of load

models built in itself, tap changers, generator models

and reactive compensation models. Therefore, it is very

important to select proper built in models to simulate the

scenario of the system in order to have an accurate detail

that matches real life scenarios and thus achieving

excellence.

V. OVERVIEW ON IEEE 14 BUS TEST SYSTEM

A mathematical model of standard 14 bus system is

created in PSS/E with 100 MVA and 69 KV as base. It

consists of 14 buses, 4 transformers, 12 static loads and

three voltage levels. The system can withstand the N-1

contingency due to which if tripping of any one of the

transmission line or one of the generating unit occures,

the system will operate normally. The dynamic file of

PSS/E has contained only the dynamic data of the

generators.

SHODH SANGAM -- A RKDF University Journal of Science and Engineering

ISSN No. 2581-5806 http://www.shodhsangam.rkdf.ac.in Vol.-03, No.-04, July-2020, Page 27

Fig. 5 IEEE 14 bus model in PSS/E

Table. I Line Data for IEEE 14 Bus System

Between

Buses

Line Impedence

Resistance (Ω) Reactance (Ω)

1-2 0.92268 2.81708

2-3 2.23719 9.42535

2-4 2.76662 8.3946

1-5 2.57237 10.6189

2-5 2.71139 8.27843

3-4 3.28557 8.14274

4-5 0.63559 2.00486

5-6 0 0

4-7 0 0

7-8 0 0

4-9 0 0

7-9 0 5.23758

9-10 1.51447 4.02305

6-11 4.522 9.46963

6-12 5.85175 12.1791

6-13 3.1494 6.20215

9-14 6.05171 12.8728

10-11 3.9064 9.14445

12-13 10.518 9.51629

Table. II Tap Setting Values for Transformers

Transformers Tap Ratio Between Buses

1 0.932 5-6

2 0.969 4-9

3 0.978 4-7

Table. III Bus Data for IEEE 14 Bus Test System

Bus

No.

Generation Load

Real Power

MW

Reactive

Power MVAr

Real Power

MW

Reactive

Power MVAr

1 232.4 -16.9 0 0

2 40 42.4 21.7 12.7

3 0 23.4 94.2 19

4 0 0 47.8 3.9

5 0 0 7.6 1.6

6 0 0 11.2 7.5

7 0 0 0 0

8 0 0 0 0

9 0 0 29.5 16.6

10 0 0 9 5.8

11 0 0 3.5 1.8

12 0 0 6.1 1.6

13 0 0 13.5 5.8

14 0 0 14.9 5

VI. SIMULATION RESULTS

Continuation power flow has been performed to

observe the effects of large scale solar PV integration on

the voltage stability. Figures below show the P-V curve

for different solar PV penetration levels. The results are

also tabulated in table 4. It can be seen from the figure 6

and 7 that for small penetration, the critical point is

nearly identical to the base case. However, as we go on

increasing the solar PV penetration levels, the voltage

stability critical point increases more and more. This indicates that by integrating more distributed

photovoltaic power plants, we can improve voltage

stability of the system. But, on the other hand, as we

increase the penetration level, the voltage level of the

buses goes on increasing and it breaches the upper

voltage limit at a certain point. Voltage profile of buses

with different penetration is also shown in figure 8 and 9.

Table. IV Changes in the Load Margin at Different Penetration Level

PV Penetration

Level Load Margin % Change

Base Case 853 MW -

5% 930 MW 9 %

10% 1018 MW 19 %

20% 1088 MW 27.5 %

SHODH SANGAM -- A RKDF University Journal of Science and Engineering

ISSN No. 2581-5806 http://www.shodhsangam.rkdf.ac.in Vol.-03, No.-04, July-2020, Page 28

Fig. 6 Power-voltage (P-V) curves for IEEE 14 bus systems without

solar

Fig. 7 Power-voltage (P-V) curves for IEEE 14 bus systems with 20%

PV penetration

Fig. 8 Voltage profile of buses without solar plants

Fig. 9 Voltage profile of buses with 20% penetration

VII. CONCLUSIONS

On the basis of research done here in this report

regarding effect of large scale integration of solar PV

plant on the power system voltage stability, following

conclusions can be made:

The maximum allowable percentage of PV that can

be integrated to the existing grid is found to be around

30%. This conclusion is based on frequency deviation in

the system under a disturbance. Also, it was found that

as the PV penetration into the grid increases, frequency

deviation also increases. An equivalent size of conventional generation was deactivated before

penetration. This frequency deviation is occurring due to

the absence of mechanical inertia which in case of

conventional generating units, always present. The

maximum allowable percentage of PV that can be

integrated to the existing grid is found to be around 30,

after further penetration the bus voltages were seen to

reach 1.1 per unit. The voltage recovery time is also

increased when PV plants are present in the grid. The

main cause of this phenomenon is the loss of reactive

power.

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